The invention relates to a bone-anchoring assembly. More particularly, the invention relates to bone anchoring elements that are capable of being pivotably attached to osteosynthesis fixation plates or longitudinal support bars for the fixation of bone segments such as vertebrae.
Implants for fixation of bones such as bone plates, longitudinal support bars, pedicle screws, and bone anchoring assemblies increasingly are used in osteosynthesis applications. Such devices are useful for treating fractures of bones, for anchoring bone segments, or for providing support to bones weakened from disease or defect.
One such implant is disclosed in the German utility model DE 297 10 979 to Aesculap. The implant comprises a bone anchoring element that is insertable into a bone segment. The bone anchoring element can then be mounted using a detachable ball clamp to a connection element. The connection element, in turn, can be clamped to a longitudinal support or to another bone anchoring element. By connecting several bone anchoring elements together, bone segments or vertebra can be rigidly connected together. As disclosed in the German utility model, the anchoring elements are in the form of hollow, cylindrical bone screws that have an external threaded and are fitted with radial boreholes located between the threads. A ball joint pivtotably supports the bone screw in the connection element until the ball joint is locked by the ball clamp. However, the drawback to the disclosed implant is the dish-shaped seat of the ballhead of the bone screw only allows inserting the bone screw unilaterally from above into the connection element and, as a result, the bone screw and connection element must be screwed jointly into the bone.
PCT publication no. WO 96/08206 to Foley discloses an osteosynthesis device with an elongated bone fixation plate and several bone anchoring elements. The disclosed bone anchoring elements consist of bone screws with spherical and radially elastic screw heads. The bone screws are inserted into the fixation plate at the desired angle and conical fixation screws are inserted into the spherical screw head of the bone fixation screw, thereby radially expanding the screw head and fixing the screw at the desired angle in the bone plate. The drawback to the disclosed osteosynthesis device is that the bone screws must be actively supported at the desired angle of insertion until they can be fixed with the conical fixation screws. As a result, the disclosed device is difficult to implant.
A similar drawback is present in the osteosynthesis device disclosed in PCT publication no. WO 88/03781 to Raveh. The disclosed osteosynthesis device comprises a bone fixation plate and several bone screws used to fix the plate to the desired bone area. The bone screws disclosed in this application are inserted into the bone fixation plate at the desired angle and conical fixation screws are then inserted into the bone screw heads to expand and lock the bone screw heads into the bone fixation plate. Again, the drawback to the disclosed osteosynthesis device is that the bone screws must be actively supported at the desired angle of insertion until they can be fixed with the conical fixation screws.
Another osteosynthesis device comprising a bone fixation plate and bone fixation means is disclosed in published EPO application no. EP 0 809 075 to Benzel. The bone fixation means disclosed comprises a bone screw with a spherical head and a hollow shaft that is configured and dimensioned to threadably receive a bone expansion screw. The bone screw is inserted through the bone fixation plate at the desired angle into the bone and the bone expansion screw is then inserted into the bone screw to expand and lock the bone screw in at the desired angle in the bone plate and the bone. However, the drawback to this device is the same as the above devices in that the bone screws must be actively supported at the desired angle of insertion until they can be fixed with the bone expansion screws.
In light of the foregoing, it is clear that there exists a need for an improved bone anchoring element.
The present invention relates to a bone anchoring assembly having at least one bone anchoring element capable of being attached to at least one osteosynthesis plate or bar for the fixation of bone segments. The at least one bone anchoring element preferably includes a circular-cylindrical body with a hemi-spherical connecting element at the posterior or upper end of the body for coupling to another fixation element, a flange located at the upper end of the body for limiting the insertion depth of the bone anchoring element, a plurality of radial borehole passages located on the body of the bone anchoring element, an external, self-tapping thread that extends over a portion of the bone anchoring element, and teeth at the anterior or lower end of the body for cutting into bone.
In one preferred embodiment, the bone anchoring element consists of a circular-cylindrical hollow body fitted at one end with a hemi-spherical connector that is mounted concentrically with the bone anchoring element""s central axis. The hemi-spherical connector is configured and dimensioned to be pivotably coupled to another fixation element such as a bone fixation plate or longitudinal support bars. Specifically, the hemi-spherical connector comprises a number of resilient blades, arranged in a circular pattern, that are radially displaceable either toward or away from the central axis of the bone anchoring element which allows the connector to be compressed or expanded thereby allowing it to snap into or out of a receiving cavity of the fixation element. Preferably, the connector blades are formed by slits running parallel to the central axis of the bone anchoring element along the periphery of the spherical connector. In addition, since the connector is shaped as a hemi-sphere and is radially deformable, the bone anchoring element can be pivotably coupled and supported in the receiving cavity of the fixation element.
The hemi-spherical connector, preferably, has a diameter D, a height of U, and a ratio of U/D that is between 0.4 and 0.7. Furthermore, the radially displaceable blades are elastically variable in a range between 0.95 to 1.05 of the diameter D. The connector can also be fixed at a particular position or angle through the use of conical fixation screws. The connector has a conically tapering bore located concentrically to the central axis of the bone anchoring element in which a conical fixation screw can be threadably received. Typically, the bone anchoring element is snapped into a receiving cavity of another implantable element and is positioned at the desired angle. A conical fixation screw is then inserted through the implantable element into the threaded bore located in the connector. As the conical fixation screw is tightened, the blades of the spherical connector are displaced outwardly locking the bone anchoring element at the desired angle to the implantable element.
In another preferred embodiment, the outside surface of the anterior or lower portion of the anchoring element does not contain an external thread allowing for a smooth surface with radial borehole passages. The radial borehole passages allow the osteoinductive material located within the hollow anchoring body to fuse with the bone located outside the anchoring body. In addition, the radial borehole passages reduce the amount of material needed to create the implant, thereby substantially lowering the total weight of the bone anchoring element.
In a further preferred embodiment, the bone anchoring apparatus comprises bone anchoring elements and plates fitted with receiving means the spherical connectors of the bone anchoring elements. The receiving means essentially consist of spherical cavities appropriately sized to allow the hemi-spherical connectors to snap into the cavity and be pivotably supported in the plate. Also, the bone anchoring element is detachably affixed to the plate by bone anchoring fasteners such as screws or nuts. Typically, the connector allows a 15xc2x0 to 35xc2x0 angular range of pivotal motion for the bone anchoring element with respect to the axis orthogonal to the plate surface. Each plate also has an elongated central channel that extends along a central axis, across most of the length of the plate, capable of receiving a fastener that will affix the plates together at any distance along the central axis within the central channel. Typically, the distance the two plates are displaceable from each other ranges between 20 mm to 60 mm. The fastener is preferably a screw or a bolt and the plates preferably have textured surfaces at their respective points of contact to prevent slippage of the plates with respect to each other and to increase the stability of the affixed plates. In addition, the plates preferably have lateral lugs to further prevent slippage and to prevent rotation of the plates with respect to each other.